1. Field of the Invention
This invention relates generally to methods and apparatus for transferring heat, mass and momentum between a fluid and a surface, and more particularly to methods and apparatus for transferring heat, mass and momentum between a fluid and a surface in which the fluid is separated into a multiplicity of tiny jets that impinge upon the surface and flow across it for very short distances before reforming jets that leave the surface.
2. Brief Description of the Prior Art
Generally, heat is transferred from a hot surface to a cold fluid (gas or liquid) by molecules that strike the surface, then rebound from it with increased thermal energy. Engineers usually apply this basic principle by allowing the fluid to flow along the surface, but the complexity of the flow defies mathematical description.
Conventionally, the fluid flows parallel to the surface and each fluid element becomes hotter and hotter as it flows across the hot surface; therefore it becomes less capable of taking heat from the surface as it proceeds because the temperature difference between the fluid and the surface decreases. Also, there is a thermal boundary layer that grows with distance from the leading edge, and the thicker the boundary layer the greater the resistance to the transfer of heat from the surface to the fluid. The laminar boundary layer at some critical distance from the leading edge makes a transition to a turbulent boundary layer. In this regime, the heat transfer rate is considerably increased. However, the further the fluid travels along the surface, opposed by wall friction, the greater the pressure drop and requirement for pump power. The physics of the problem is such that engineers use mean values and empirical relations to design such systems.
U.S. Pat. Nos. 4,471,000, 4,489,506, and 4,956,271 assigned to Wolverine Corportation of Merrimac, Mass., disclose apparatus utilizing tubes to direct jets of air on or into a medium and then suck the exhaust back through spaces between the tubes. The incoming jets merely diffuse through the spaces between the particles in the treated medium and the surfaces of those particles are treated by the molecules that strike it due to the turbulent diffusion. The present invention, on the other hand, establishes thin laminar boundary layers having high heat, mass, and/or momentum transfer rates, and high efficiency, thus requiring much less fan power because the pressure drop associated with those laminar boundary layers is at a minimum.
U.S. Pat. Nos. 4,201,499 and 4,776,107, also assigned to Wolverine Corporation of Merrimac, Mass., disclose apparatus utilizing tubes to direct jets of gas onto a medium wherein the flow from the tubs is not constrained to flow for any appreciable distance along the surface of the medium before it turns away and flows back between the tubes. This is a rather inefficient use of the gas, compared to the present invention wherein inlet jets emerging from a surface that confines the flow of the jets along the treatment surface which is in close proximity. The present invention also allows the flow from neighboring jets to interact on the surface to form outlet jets, and the length of those flows is always large compared to the diameter of the jets, both inlet and outlet. This is what assures high efficiency of heat, mass, and/or momentum transfer.
The present invention is distinguished over the prior art methods in general by a method for transferring heat, mass and momentum between a fluid and a surface. Instead of the usual approach in which the fluid is made to flow parallel to the surface, the present apparatus and method separates the fluid into a multiplicity of tiny jets that impinge upon the surface and flow across it for very short distances before reforming jets that leave the surface. While in contact with the surface, the fluid flow is laminar and the boundary layer that resists transfer of heat, mass and momentum is extremely thin. Hence, heat, mass and momentum transfer coefficients are large and predictable from first principles of physics. The pressure drop and fan power required to form these jets are generally less than that required to drive the flow parallel to the surface for long distances, where the boundary layer grows much thicker. The present method and apparatus has applications in a variety of fields including; fluid/surface heat transfer, fluid/fluid heat transfer, surface propulsion, surface levitation, surface skin drag and skin heating reduction, surface drying, surface cleaning, hair drying, ice melting, surface coating, surface chemical reactions, surface phase-change reactions, clean efficient coal combustion, air pollution control, and particle separation from fluids.